Further arguments are proposed to show that the 'size effect' observed in the competition between acleavages, as well as in the competition between losses of alkyl and alkane from aliphatic amines, can be interpreted as a 'rotational effect'. # 1998 John Wiley & Sons, Ltd. Received 3 November 1997; Revised 19 November 1997; Accepted 20 November 1997 Rapid Commun. Mass Spectrom. 12, 57-60 (1998 It has been now widely recognized that, in the unimolecular decomposition of isolated organic ions, the incipient ion (I) and neutral (N) fragments, before separation, can mutually reorientate while remaining electrostatically associated by ion-neutral interaction.1-12 It has been proposed that a critical energy (a so-called 'I-N reorientation threshold'), above which the two fragments can separately rotate is associated with this process. I-N reorientation may open to the parent ion new reaction pathways not accessible from the original structure, otherwise it is only a prelude to simple decomposition. In any case, when ion-neutral reorientation is irreversible it is a crucial event because, as a consequence, the fate of a given parent ion is actually decided as soon as incipient ion and neutral fragments have started mutual reorientation. Thus, the competition between reactions may be governed not only by their relative decomposition critical energies, but also by their respective reorientation critical energies.11,12 Therefore, the investigation of the factors that can influence the reorientation of incipient ionic and neutral fragments is important.Two effects have been shown 11,12 to play a significant role in this matter: an electrostatic effect and a rotational effect.When the neutral contains a permanent dipole, ionneutral reorientation leads to a more or less stable reoriented structure in which the charge-dipole distance is minimized and whose stabilization energy may be estimated, given an ion-dipole distance, as a function of the dipole moment and the polarizability of the neutral. When the neutral has no permanent dipole, only the contribution involving the polarizability is efficient in the electrostatic effect. In particular for a homologous series of neutrals the polarizability increases with the size. In both cases, the reorientation critical energy normally obeys the Marcus rule, 13 i.e. reorientation is more facile when leading to a more stable product.A rotational effect is expected when the initial rotational energy of the parent ion and the translational energy of separation of the incipient fragments participate in the relative rotational motion of the latter. These contributions also depend on the size of the fragments and may overshadow the participation of polarizability in the electrostatic effect.Some examples of the rotational effect have been described in the electron induced fragmentation of low energy alkanes 14 and aliphatic amines. 11,12 Further arguments may now be proposed that confirm the important role played by the individual rotation of the fragments during ion-neutral separation.
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